In many practical situations in which compounds are subjected to polymerization or where adhesives are used, it is desirable to have polymerizable compositions and adhesive compositions which can cure on demand. Cure on demand means that the polymerization can be initiated when desired. A significant problem with cure on demand compositions is the stability of the compositions. Many such compositions will begin curing at, or near, ambient temperature or will partially cure at ambient temperature resulting in an increased viscosity causing difficulties in handling and reduced functionality of the polymerizable composition or adhesive composition.
Low surface energy olefins such as polyethylene, polypropylene and polytetrafluoroethylene have a variety of attractive properties in a variety of uses, such as for toys, automobile parts, furniture applications and the like. Because of the low surface energy of these plastic materials, it is very difficult to find adhesive compositions which bond to these materials. The commercially available adhesives which are used for these plastics require time consuming or extensive pretreatment of the surface before the adhesive will bond to the surface. Such pretreatments include corona treatment, flame treatment and the like. The requirement for extensive pretreatment of the surface results in significant limitations to the designers of automobile components, toys, furniture and the like. It is desirable to have adhesive compositions which are capable of bonding to low surface energy substrates, and bonding low surface energy substrates to other substrates, without the need for extensive or costly pretreatment.
Organoborane compounds exhibit good adhesion to low surface energy olefins. Organoborane compounds are, however, pyrophoric in air and are very reactive in methacrylate adhesive systems. Organoborane compounds can be stabilized by complexing them with a blocking agent. The blocking agents prevent pyrophoric reactions of the organoborane compounds as well reducing curing of compositions at or near ambient temperatures before desired.
Sonnenschein et al, U.S. Pat. No. 6,806,330 discloses amine organoborane complex polymerization initiators and polymerizable compositions. The amine acts as a blocking agent to stabilize the compositions. The amine is an amine having an amidine structural component; an aliphatic heterocycle having at least one nitrogen in the heterocyclic ring wherein the heterocyclic compound may also contain one or more nitrogen atoms, oxygen atoms, sulfur atoms, or double bonds in the heterocycle; a primary amine which in addition has one or more hydrogen bond accepting groups wherein there is at least two carbon atoms, preferably three, between the primary amine and the hydrogen bond accepting group, such that due to inter- or intramolecular interactions within the complex the strength of the B—N bond is increased; or a conjugated imine. Preferred hydrogen bond accepting groups include the following: a secondary amine, a tertiary amine, an ether, a halogen, a polyether group or a polyamine group. The complexes of the invention are used in polymerizable compositions, adhesive compositions and coatings compositions containing compounds having moieties which polymerize under free radical polymerization conditions.
Mottus et al., U.S. Pat. No. 3,275,611 discloses a process for polymerizing olefinic compounds with a catalyst comprising an organoborane compound, a peroxygen compound and an amine. It is disclosed that the organoborane compound and amine may be added to the reaction mixture separately or as a preformed complex, and that the complex is preferred. The presence of the amine in the complex reduces the pyrophoricity of the organoborane in air. Among the amine complexing agents disclosed are pyridine, aniline, toluidine, dimethylbenzylamine, and nicotine. Many of the complexes disclosed in Mottus are pyrophoric at all amine to boron atom ratios. In addition, many of the amine complexes do not display significant adhesive properties when applied to low surface energy substrates.
A series of patents issued to Skoultchi (U.S. Pat. Nos. 5,106,928, 5,143,884, 5,286,821, 5,310,835 and 5,376,746) disclose a two-part initiator system that is reportedly useful in acrylic adhesive compositions. The first part of the two-part system includes a stable organoborane-amine complex and the second part includes a destabilizer or activator such as an organic acid or an aldehyde. The organoborane compound of the complex has three ligands which can be selected from C.sub.1-10 alkyl groups or phenyl groups. Useful amines disclosed include octylamine, 1,6 diaminohexane, diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine, 1,3 propylene diamine, and 1,2 propylene diamine. The adhesive compositions are disclosed to be useful in structural and semi-structural adhesive applications, such as speaker magnets, metal to metal bonding, automotive glass to metal bonding, glass to glass bonding, circuit board component bonding, bonding select plastics to metal, glass to wood, etc. and for electric motor magnets.
Zharov et al. discloses in a series of U.S. Pat. Nos. (5,539,070; 5,690,780; and 5,691,065) polymerizable acrylic compositions which are particularly useful as adhesives wherein organoborane amine complexes are used to initiate cure. The organoboranes used have three ligands attached to the borane atom which are selected from C.sub.1-10 alkyl groups and phenyl. The amine is an alkanol amine or a diamine where the first amine group can be a primary or secondary amine and the second amine is a primary amine. It is disclosed that these complexes are good for initiating polymerization of an adhesive which bonds to low surface energy substrates.
Pocius in a series of patents (U.S. Pat. Nos. 5,616,796; 5,621,143; 5,681,910; 5,686,544; 5,718,977; and 5,795,657) disclose amine organoborane complexes with a variety of amines such as polyoxyalkylene polyamines and polyamines which are the reaction product of diprimary amines and compound having at least two groups which react with a primary amine.
Many of the complexes disclosed in the Zharov, Skoultchi and Pocius Patents are not stable in compositions containing olefinic unsaturation at, or near, ambient temperatures and thus the complexes disassociate and induce polymerization at, or near, ambient temperature with time. This instability at, or near, ambient temperature can result in polymerization before desired and can result in compositions which are unsuitable for the desired use.
Therefore, there is a desire for an improved blocking agent that can be used in an organoborane complex for use in initiator systems for free radical polymerization which are safe to handle, not pyrophoric, which can be used to form cure on demand polymer systems or can be used in adhesive systems which cure on demand. Such improved blocking agents can also be used in adhesive systems which are capable of bonding to low surface energy substrates, and initiator systems which facilitate such bonding. In addition to such needs, the complexes need to be thermally stable, that is do not disassociate at, or near, ambient temperatures and thereby initiate polymerization before desired. What is further desired is an improved blocking agent for an organoborane complex that can be used in polymer compositions and adhesive systems which are thermally stable at, or near, ambient temperatures and which will undergo polymerization when the user desires.